The present invention relates to a circuitized substrate, and particularly to such a substrate designed especially for high-frequency applications.
Circuitized substrates comprised of an insulating material provided with conductive tracks defining an electric circuit, are commonly used in several electronic assemblies. For example, a circuitized substrate may be used as a chip carrier for protecting an electronic circuit integrated in a chip of semiconductor material from mechanical stresses, the chip being electrically coupled to the carrier. Moreover, a circuitized substrate may also be used as a Printed Circuit Board (PCB) for having multiple electronic modules and components mounted thereon, including the aforementioned chip carrier.
In some applications, the circuitized substrate has a multi-layer structure, with a plurality of conductive layers (tracks) insulated from each other and via-holes used to connect the conductive layers electrically where necessary. The transmission of an electric signal on the corresponding tracks and via-holes generates an electromagnetic wave which propagates along a transmission line defined by a dielectric material surrounding the tracks and the via-holes. When the electronic assembly embedding the circuitized substrate works at a high frequency (for example higher than 1 GHz), the propagation of this wave (microwave) may be adversely affected by the performance of the electronic assembly as a whole. Particularly, a discontinuity (or transition) in the transmission line, such as a change in structure, material properties and/or design features, may generate a reflected wave. Moreover, the electronic assembly includes stray structures (capacitors, inductors and resistors), which may act as low pass filters for the transmitted signal. As a consequence, the integrity of the electromagnetic wave propagated along the transmission line may be affected. For example, a signal switching between a low voltage (logic value 0) and a high voltage (logic value 1) is generated as a square-shaped wave by a driver unit; because of the discontinuities and stray structures associated with the transmission line, the signal is generally received as a pseudo-sinusoidal wave by a different unit.
The quality of the transmission can be visualized by a so-called xe2x80x9ceye diagramxe2x80x9d, which plots the value of the received signal as a function of the phase of a clock signal controlling the switching of the transmitted signal. The above described discontinuities and stray structures associated with the transmission line may reduce the opening of the eye diagram; therefore, it is quite difficult to understand if a switching transition has actually taken place or if the shift of a signal baseline is due to a background noise. Moreover, the received signal features a slow ramp for each switching transition of the transmitted signal. It is then possible that the received signal remains at a stable value for a brief time, even shorter than a settling time needed for recognizing the switching transition. As a consequence, the switching transition may not be correctly distinguished from a sudden noise pulse (spike).
These drawbacks are particular acute in modern electronic assemblies working with a reduced level of a power supply voltage (down to 1.2 V). In this case, there is a very low margin to discriminate between the logic value 0 (0V) and the logic value 1 (1.2V).
Therefore, the low quality of the transmission forces the electronic assembly to operate at a frequency far lower than the working frequency that is affordable by an active component of the electronic assembly (such as the electronic circuit integrated in the chip).
It is believed, therefore, that a circuitized substrate which overcomes the above disadvantages would constitute a significant advancement in the art.
It is, therefore, a primary object of this invention to enhance the circuitized substrate art.
It is another object of the invention to provide a circuitized substrate which overcomes the disadvantages of known substrates such as discussed above.
It is yet another object of the invention to provide a circuitized substrate which can be expeditiously produced, thus assuring a relatively low cost for same.
According to one aspect of the invention, there is provided a circuitized substrate comprising a plurality of electrically conductive layers, selected ones of the electrically conductive layers electrically insulated from adjacent ones of the electrically conductive layers by dielectric material, at least one electrically conductive via-hole extending between and electrically coupling two non-adjacent ones of the electrically conductive layers and adapted for transmitting a high frequency signal therebetween, at least one of the electrically conductive layers being an intermediate conductive layer positioned between the two non-adjacent ones of the electrically conductive layers and including at least one shielding track connectable to a reference voltage, the shielding track substantially surrounding the at least one electrically conductive via-hole.